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[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]]

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[[!tag open_issue_xen]]

IRC, freenode, #hurd, 2011-09-01:

    <youpi> hum, f951 does myriads of   71->io_seek_request (32768 0) = 0 32768
    <youpi> no wonder it's slow
    <youpi> unfortunately that's also what it does on linux, the system call is
      just less costly
    <youpi> apparently gfortran calls io_seek for, like, every token of the
      sourced file
    <youpi> (fgetpos actually, but that's the same)
    <youpi> and it is indeed about 10 times slower under Xen for some reason

IRC, freenode, #hurd, 2011-11-02:

    <youpi> btw, we have a performance issue with xen
    <youpi> an lseek() call costs a huge lot
    <youpi> like 1ms
    <youpi> while the same costs just a few dozens µs with kvm
    <youpi> there's of course the cost of switching between ring3, ring0,
      ring1, ring0, ring3, but still
    <gianluca> oh, nice.
    <youpi> lseek is supposed to perform only a back&forth
    <youpi> and I don't observe disk activity, so it's not waiting for the disk
      to complete whatever atime change & such :)
    <youpi> it was mentioned that perhaps xen in hvm mode with pv drivers would
      be faster
    <youpi> thanks to the ring3/"1" switching  being done by the processor
    <youpi> (and assuming npt)
    <gianluca> hm
    <gianluca> i'll look into that, sounds fun.
    <gianluca> :)
    <tschwinge> Here is a testcase:
      http://www.gnu.org/software/hurd/open_issues/performance/io_system/binutils_ld_64ksec.html

[[performance/io_system/binutils_ld_64ksec]].

Also see the simple testcases [[test-lseek.c]] and [[test-mach.c]].

IRC, freenode, #hurd, 2011-11-05:

    <youpi> [test-mach.c is] mostly as a reference for the trap overhead
    <youpi> 0.56µs (xen) vs 0.48µs(kvm) on test-mach
    <youpi> 455µs(xen) vs 16µs(kvm) on test-lseek
    <youpi> that might simply be an issue in the RPC  mechanism, which behaves
      badly with the xen memory management
    <youpi> yes, about 0.5ms for an lseek, that's quite high :)